Molecular structure of human TFIIH

TFIIH is a multiprotein complex required for both transcription and DNA rep air. Single particles of human TFIIH were revealed by electron microscopy a nd image processing at a resolution of 3.8 nm. TFIIH is 16 x 12.5 x 7.5 nm in size and is organized into a ring-like structure from which a large prot ein domain protrudes out. A subcomplex assembled from five recombinant core subunits also forms a circular architecture that can be superimposed on th e ring found in human TFIIH. Immunolabeling experiments localize several su bunits: p44, within the ring structure, forms the base of the protruding pr otein density which includes the cdk7 kinase, cyclin H, and MAT1. Within th e ring structure, p44 was flanked on either side by the XPB and XPD helicas es. These observations provide us with a quartenary organizational model of TFIIH.